Conventionally, an image forming apparatus that uses an electrophotographic image process employs a process cartridge system in which a developing unit, a photosensitive drum and process means for acting upon the photosensitive drum are integrated into the form of a cartridge and the cartridge is capable of being removably installed in the main body of the image forming apparatus. In accordance with this process cartridge system, maintenance of the image forming apparatus can be performed by a user himself without relying upon a serviceman, thereby greatly enhancing operability. For this reason, such an image forming apparatus is in wide use.
However, because the amount of toner that can be contained in the cartridge is limited, this process cartridge system has certain drawbacks.
Specifically, when an image forming apparatus has a high frequency of use, a process cartridge system with a small toner capacity requires that the cartridge be replaced very frequently and, as a result, usability is poor. Further, when the cartridge is replaced, not only the toner but also the cartridge components (photosensitive drum and charge roller, etc.) and developing unit are replaced in their entirety. This invites an increase in running cost.
Accordingly, a toner supply system in which the developing unit, etc., is placed on the side of the main body of the image forming apparatus and use is made of a so-called toner supply cartridge that merely supplies toner from the cartridge has become the most widely employed system in equipment such as copiers that have a high frequency of use. A disadvantage of this toner supply system is that maintenance of the image forming apparatus such as replacement of consumable parts is almost impossible for the user to perform. This makes maintenance by a serviceman necessary and increases the burden upon the user.
A toner-supply-type process cartridge that has the advantages of both the process cartridge system and toner supply system has been proposed in recent years. This system comprises at least a process cartridge, which has a developing unit for visualizing by toner an electrostatic latent image formed on an image carrier (photosensitive drum), and a toner supply unit for supplying the developing unit with toner. By removably installing these two units in the main body of an image forming apparatus, maintenance and usability are improved and running cost is lowered.
Such a toner supply unit has means for sensing the amount of toner in the toner supply unit and, based upon the result of sensing, is capable of notifying the user of the fact that the toner supply unit should be replaced. An example of means of sensing the amount of toner in the toner supply unit is (A) an arrangement in which the toner supply unit is provided with various well-known sensors, such as by providing the inner wall of the vessel with a piezoelectric sensor and sensing the electrostatic capacitance of the toner, and in which a light-transmitting system, etc., is used. Further, as described in Japanese Patent No. 2575404, (B) there is a method in which means for sensing the mixing ratio (toner concentration) of, e.g., a two-component developer consisting of toner and carrier is provided on the side of the developing unit that is supplied with the toner. When the output of the toner concentration sensing means is equal to or greater than a fixed value (i.e., when the toner concentration is below a fixed value), it is sensed that there is no toner in the toner supply unit on the grounds that toner is no longer being supplied. Further, (C) there is a method which relies upon toner consumption information as from the output of a toner concentration sensor or from image dot information, wherein the total amount of toner that has been supplied is estimated by adding up and storing the number of revolutions of a toner supply screw of the toner supply unit and the absence of toner is sensed based upon the result of the estimation.
In the case of (A), the fact that the toner supply unit is provided with sensing means for sensing the amount of toner directly makes it possible to give an indication of remaining amount of toner during usage and to notify of the absence of toner at an appropriate timing. However, a problem is that the cost of the toner supply unit, which is the replaceable unit, rises by an amount commensurate with the sensing means provided.
Further, in the case of (B), notification of absence of toner can be given at the stage at which the toner supply unit truly runs out of toner. However, an indication of remaining amount of toner during usage cannot be presented and it is also not possible to give an early warning indicating approximately how much longer the toner supply unit can be used until replacement is required. Further, in the case of (C), which is the converse of (B), the amount of consumed toner can be estimated by adding up and storing the number of revolutions of the toner supply screw, which has a strong relationship to amount of consumed toner. This is extremely effective in that the amount of toner remaining in the toner supply unit during usage can be displayed in stages and the user can be notified of the timing for replacement of the toner supply unit. However, at the stage where absence of toner is determined, an error with respect to the actual amount of remaining toner can occur and there is the danger that the user will be alerted in advance to replace the toner supply unit regardless of the fact that toner still remains. By combining (B) and (C), an indication of amount of remaining toner can be presenting during usage and the user can be notified of absence of toner at the stage where the toner supply unit has actually run out of toner. In this case, however, a discrepancy develops in the period of time from estimation of absence of toner to sensing of the fact that toner has actually run out. For example, once an indication that the amount of remaining toner is 1% is given, it may be a long while before toner actually runs out and printing becomes impossible. On the other hand, printing may become impossible in a short period of time following the indication. Thus there is the possibility that the user will become confused.